The present invention relates generally to braking systems and, more specifically, to brake actuators. Even more particularly, the present invention relates to a snap-fit tamper-resistant spring brake actuator and a method of assembling the same.
Fluid-operated braking systems have long been used to control the movement of motor vehicles in a safe and effective manner. In particular, air brakes are commonly used on commercial vehicles such as trucks, which typically have large gross vehicle weights. The considerable inertial mass of these heavy-duty vehicles in combination with the high speeds at which they travel requires a braking system which responds rapidly with substantial braking power. One system component which is instrumental in the operation of air brake systems is the brake actuator, which provides the force necessary when braking a vehicle.
In a typical double diaphragm spring brake actuator, a barrel-shaped power spring stores potential energy and exerts the large force required for braking in case of air pressure failure. Air pressure acting on a diaphragm compresses the power spring and maintains it in its brake release position. When the air is exhausted, the power spring acts on the diaphragm, typically an elastomeric diaphragm or a piston, and thereby applies the brakes in case of failure of the system air pressure.
The power spring is positioned in a spring chamber, which is typically formed by clamping an elastomeric diaphragm between a head (sometimes also known as a spring housing or spring chamber) and a flange case (sometimes known as an adaptor). The power spring is compressed within the spring chamber between the head and the diaphragm. The power spring has a high spring constant and is normally compressed to a height of less than 3 inches from an original uncompressed heightof from 9 to 12 inches. The power spring, therefore, stores a substantial amount of potential energy, usually exerting a force on the head of about 2,000 to 3,000 pounds.
Unauthorized removal of the head, therefore, could be dangerous due to the large potential energy of the power spring. In addition, unauthorized replacement of the head could result in a poor diaphragm seal or other internal defects. Therefore, it is conventional to discourage unauthorized removal and replacement of the head by attaching a warning label and/or providing a securing means which is tamper-resistant or at least tamper-evident, such that a new securing means, not readily available to unauthorized personnel is required before the head can be reattached.
Various approaches have been heretofore proposed for securing the brake actuator head to the flange case to prevent improvident disassembly of the two parts. GB Patent No. 2,000,225, for example, discloses in one embodiment a brake actuator including a head secured to a flange case with a circlip. The circlip is seated in an inwardly facing groove of the flange case and extends over an annular lip of the head, such that the head can only be separated from the flange case by deforming or destroying the flange case. The circlip, however, is accessible from between the flange case and an edge of the head and, therefore, could possibly be tampered with. Moreover, the design is complex, and therefore difficult and costly to manufacture and assemble.
The ""225 patent, in another embodiment, also discloses a brake actuator including a head having a lower edge that is spun, or inelastically deformed, over a flange case, such that the head can only be separated from the flange case by deforming or destroying the lower edge of the head Similar arrangements are also disclosed in U.S. Pat. Nos. 4,850,263, 4,960,036, 5,067,391, 5,205,205, 5,263,403, 5,311,809, 5,433,138, and 5,640,894. A disadvantage of each of these prior art references is that special deforming machinery is needed to assemble the brake actuator, making assembly both costly and difficult. Another disadvantage is that the deformed lower edge of the head can easily be undeformed by an unauthorized person attempting to disassemble the brake actuator.
U.S. Pat. No. 5,285,716 discloses a brake actuator having a head and a flange case welded together in a tamperproof manner. As disclosed in the patent, however, it is a relatively easy task to grind away the weld to remove the head. In addition, the flange case must disadvantageously be made of steel in order to be welded to the steel head. Flange cases are normally made of aluminum.
U.S. Pat. Nos. 5,315,918 and 5,353,688 both disclose, in one embodiment, a brake actuator including a head screwed onto a flange case and secured with an insert or a pin creating an interference fit. The insert or pin may be welded in place. In another embodiment of the ""918 and ""688 patents, and in U.S. Pat. No. 4,887,513, a brake actuator is disclosed having a head which is joined to a flange case with a bayonet connection. U.S. Pat. No. 5,560,280 discloses a spring chamber head and adapter head, which include a plurality of cooperating peripheral lugs and flanges thereon. The flanges are held in registration with the lugs by the compression of the periphery of the spring brake diaphragm between the heads. A disadvantage of each of these prior art references is that the design is complex, and therefore difficult and costly to manufacture and assemble. A related disadvantage is that special and complex machinery is needed to assemble the brake actuator, making assembly even more costly and difficult.
What is desired, therefore, is a spring brake actuator which is tamper resistant, which is simple in design as compared with knownspring brake actuators, which is relatively easy and inexpensive to produce and assemble, which does not require special machinery to assemble, and which can be made of conventional materials.
Accordingly, it is an object of the present invention to provide a spring brake actuator which is tamper resistant.
Another object of the present invention is to provide a spring brake actuator having the above characteristics and which is simple in design as compared with known spring brake actuators.
A further object of the present invention is to provide a spring brake actuator having the above characteristics and which is relatively easy and inexpensive to produce and assemble.
Still another object of the present invention is to provide a spring brake actuator having the above characteristics and which does not require special machinery to assemble.
Yet a further object of the present invention is to provide a spring brake actuator having the above characteristics and which can be made of conventional materials.
These and other objects of the present invention are achieved by provision of a spring brake actuator having a flange case and a head. The flange case includes a radially outwardly extending annular flange, and the head includes a shoulder and a collar extending therefrom. The collar includes a plurality of radially spaced apart slots defining a plurality of radially spaced apart tabs, each of the tabs having a hook portion projecting radially inwardly, and each of the tabs being elastically deformable radially outwardly. In an assembled position, when the head is attached to the flange case, the annular flange of the flange case is received by the collar of the head and the hook portions of the tabs engage a lower surface of the annular flange so as to inhibit separation of the head from the flange case.
Preferably, the spring brake actuator also includes a continuous retaining wire disposed around the head in an assembled position, the retaining wire inhibiting the tabs from deforming radially outwardly so as to inhibit separation of the head from the flange case. In one embodiment, the retaining wire is positioned to abut the shoulder of the head adjacent to the collar. In another embodiment, the collar includes a peripheral annular groove therein, and the retaining wire is positioned in the annular groove.
It is also preferable if the hook portion of each of the tabs includes a barb which engages a corresponding annular recess in a mating surface of the annular flange of the flange case so as to further inhibit separation of the head from the flange case. In addition, it is preferable if the hook portion of each of the tabs includes a tapered outer lower edge which contacts the annular flange during assembly so as to promote radially outward flexing of the tabs during assembly.
The spring brake actuator also includes a spring diaphragm having a periphery, which is clamped in a fluid tight manner between the annular flange of the flange case and the collar of the head.
Moreover, the radially spaced apart slots preferably extend axially through the shoulder of the head and partially up a sidewall of the head, so as to promote radially outward flexing of the tabs during assembly. When such is the case, the force required to achieve radially outward flexing of the tabs during assembly is most preferably variable depending at least in part upon the extent which the radially spaced apart slots extend axially up the sidewall of the head.
In another aspect, the present invention relates to a method of assembling such a spring brake actuator.
The invention and its particular features and advantages will become more apparent from the following detailed description considered with reference to the accompanying drawings.